A reclusive novelist, a loan shark who buries people alive, and a detective who doesn’t know who he really is. Full spoilers.
This 10-episode series, released worldwide on August 28, grew out of the Korean folktale “The Fieldmouse That Ate Fingernails” — the story of a mouse that eats a man’s clipped nails and becomes him. Every Korean grows up knowing this folktale. That’s exactly why the Korean title, Deuljwi (“field mouse”), describes the show more precisely than its English title, Mousetrap, ever could. Ryu Jun-yeol (Moonjae), Sul Kyung-gu (Noja), and Lee Kyu-hyung (Sunyong) lead this series. It’s an intelligent thriller that keeps asking you who the real villain is until the very end.
Everything from here on is a full spoiler. Haven’t watched yet? Start with the no-spoiler guide first — it’ll make the show even more fun:
Mousetrap: What You Need to Know Before You Watch.
Moonjae Was Already Living Through Other People
Moonjae is a successful novelist who hit it big early in his career, but hasn’t left his apartment in three years. He’s had panic disorder since a childhood accident — his mentally unstable mother drove into the sea with his father still in the car — and he hands over every practical part of his life to his college friend and accountant, Suhyun: his banking, his prescriptions, even the deed to his own apartment.
One day the fingerprint sensor on the phone he uses for banking stops working. Suhyun comes to see him and hands him a new phone. That same day, Detective Sunyong gets an anonymous tip about an unidentified woman’s body pulled from the Han River — not a suicide, the tip insists — along with a name, Kim Yeong-geun, a photo, and Moonjae’s address. Sunyong pays Moonjae a visit, meets him for the first time, and finds nothing.
Then the new phone’s fingerprint sensor fails too. His banking locks up. Suhyun goes silent. His apartment, it turns out, has already been sold under Suhyun’s name. He tracks her down to a class reunion happening that day — where his old classmates already believe a “fake” who’s been showing up to reunions for the past two years is the real Moonjae. On top of the absurdity, both his resident registration number and his fingerprints were re-registered by someone impersonating him two months earlier. Officially, Moonjae doesn’t exist anymore.
On the street, Kim Yeong-geun finds him — a fixer who handles rough jobs, nicknamed “Noja” because he buries people alive and hands them burial money on the way down. He currently works finding people for a gangster named Seunggyu, and he’s convinced Moonjae embezzled the startup money he’d set aside for his own fresh start and vanished with it three years ago. He beats Moonjae half to death with a bat the moment he finds him.
Moonjae escapes into an alley — and comes face to face with someone who looks exactly like him. The Rat.
Noja, and an Unlikely Alliance
On the run from Noja, Moonjae tries to track down Suhyun. It turns out that three years ago, it was Suhyun who talked him into embezzling Noja’s money in the first place. Separately, Noja learns that Seunggyu’s crew has been killing people using stolen homeless identities and dumping the bodies as unidentified corpses — and starts to suspect a connection to Moonjae’s three missing years.
Meanwhile, the Rat is watching Moonjae’s every move from a private house — where Suhyun’s young daughter is being held hostage.
Moonjae goes to Noja’s office himself. Noja starts beating him again, but Moonjae makes him an offer: help find the impostor, and he’ll return the embezzled money. Noja accepts, and the unlikely alliance begins.
Suhyun’s Final Act
Noja discovers a tracker planted on Moonjae’s phone and uses it to fake Moonjae’s death and lure Suhyun out. Cornered, Suhyun is pushed by the Rat toward suicide, and before it happens, tells the two men what she knows. Two years earlier, someone calling himself “the Rat” threatened to report Moonjae’s embezzlement, and more recently kidnapped her daughter to force her to hand over everything Moonjae had.
In front of both men, she jumps into the Han River, telling Moonjae, “this is all because of your lie.” Her daughter is found safe, but CCTV footage shows a man with Moonjae’s face at the scene, and police name the real Moonjae their prime suspect.
Chasing Seo Yumin
What Suhyun told them leads Moonjae to remember Seo Yumin, a friend from his college literature club. Noja recognizes Yumin’s photo the moment he sees it — years ago, he was the one who caught Yumin, then on the run from unpaid debts, and handed him over to Seunggyu.
The two travel to Yumin’s home island but are repeatedly ambushed by unidentified pursuers — sent by Seunggyu, who’s worried that if Noja gets caught, he’s finished too. Meanwhile, Sunyong finds a photo of a woman — Minyoung — at Yumin’s old house. Her ankle tattoo matches the Han River body, and he secures CCTV footage of a man with Moonjae’s face killing Minyoung and dumping her in the river.
Noja tracks down Seunggyu and asks what happened to the man he once handed over. Seunggyu tells him Yumin sold his own identity and then killed himself. Noja confirms it at a columbarium holding unclaimed remains under Yumin’s name.
The Fake Moonjae Completes His Theft
The fake Moonjae publishes a new novel titled The Rat, completing his social takeover of Moonjae’s identity, then preemptively reports to police that an impostor has been stalking him. The real Moonjae is left with no one left to vouch for him.
He turns to his editor, the only person who might still be able to tell real from fake. The meeting leaves her suspicious of the Rat for the first time. Separately, Sunyong’s investigation confirms how Yumin disappeared: he used a homeless man’s body in Seunggyu’s identity-laundering scheme, then resurfaced much later wearing Moonjae’s face.
A Third Moonjae, Hidden All Along
Digging through his own network, Noja turns up an old newspaper photo of Moonjae’s parents — with a boy of about five standing next to them, taken before Moonjae was even born. It also turns out Moonjae has an uncle: Han Huisu. If they can find him, he could help Moonjae prove who he really is. The two decide to track him down at his nursing home.
A pursuer catches up to them again. In the fight, Moonjae is shot. Amid the chaos, Noja is caught by police while one of Noja’s own contacts pulls Moonjae to safety. Wounded and mid-treatment, Moonjae is attacked by Seunggyu’s men and barely escapes. The police car transporting Noja is flipped by the pursuer’s vehicle; while he kills the officers inside, Noja slips away.
Sunyong, learning about Huisu independently, goes to see him — but Moonjae has already been there. Huisu has Alzheimer’s and can’t recognize anyone. The moment he sees Sunyong, though, he calls him “Moonjae.”
Sunyong turns out to be an orphan who was adopted at five by Moonjae’s parents and given the name Je Moonjae — then returned for adoption once the couple had a biological son, who was given the same name. The boy in the old photo was Sunyong. He finally understands why he’s been tangled up in this case from the start. It later becomes clear that Yumin knew about Sunyong’s past from the very beginning — the anonymous tip that opened the case was sent specifically to make sure Sunyong would be the one investigating.
Moonjae’s Truth, in the Interrogation Room
Arrested while fleeing in a stolen car, Moonjae tells Sunyong everything — his version of it, anyway. Yumin, he says, was a fake college student who wormed his way into the literature club to get close to him, then secretly read through Moonjae’s notebook of writing and story outlines. Both men liked the same woman, Minyoung, and had a huge falling-out over her on a beach in Jeju. Afterward, Yumin disappeared for a while — then resurfaced wearing Moonjae’s face. Sunyong believes him, and asks his fellow detectives to help sneak Moonjae out of custody.
At the same time, Noja — handed over to Seunggyu by the pursuer — is about to be buried alive, exactly the way he’s done to others for a living. He begs. Seunggyu covers him with dirt anyway. Elsewhere, the pursuer digs a hole to bury Huisu, but he manages to hide, and Seunggyu’s men go looking for him.
Uncle Huisu’s Whereabouts, and Total Chaos
Sunyong checks university records for Yumin, Minyoung, and Moonjae — and finds the exact opposite of what Moonjae told him. Moonjae was never actually enrolled there at all. Now certain Moonjae lied to him, Sunyong tries calling his team to stop things — only to learn Moonjae has already escaped, using the very help Sunyong arranged for him.
On the run, Moonjae is caught by one of Seunggyu’s men, who’s also hunting for Huisu — but Noja, having clawed his way out of the ground he was just buried in, kills the man and saves him. Seunggyu, sensing betrayal among his own men, kills the one who remains and ends up alone.
Everyone converges on an abandoned building in the mountains, still looking for Huisu: Moonjae and Noja, the pursuer and his crew, Sunyong and his partner, and another homicide detective. A full brawl breaks out; Moonjae and Noja slip away in the chaos. Sunyong spots Huisu mid-fight and chases him, takes a serious hit from the pursuer, and barely survives thanks to his partner — but loses Huisu again. As the police dragnet closes in, Moonjae and Noja split up and flee in different directions.
All the Secrets Come to Light
The Rat books Moonjae a flight for the next day and arranges to meet at the pension house in Jeju where he and Yumin once had their big falling-out in college. It’s a trap — the plan is to get Moonjae arrested there for murder. The Rat, meanwhile, is actually on a cruise ship.
Sunyong wakes up in the hospital, tells his team to find the Rat and Moonjae, and goes after his uncle himself. Just as he expected, he finds Huisu at the amusement park where Sunyong himself once went missing as a child. Moonjae, meanwhile, catches on to the trap and knocks on the Rat’s door aboard the cruise ship instead.
Yumin’s Truth, and His Final Moments
Knife drawn, Moonjae demands to know why Yumin did any of this. Now Yumin tells the truth — the exact opposite of the story Moonjae gave Sunyong. Yumin’s writing talent was the real thing all along, and Moonjae envied him for it. On a trip the three of them took to Jeju, Minyoung revealed she was dating Yumin — and Moonjae and Yumin had a massive falling-out that same day. Moonjae then spread false rumors about the two of them on an anonymous campus board. He had driven the woman Yumin loved to her death, then stolen his future too — winning a rookie award with a novel stolen straight from Yumin’s notebook, and becoming a star writer off the back of it.
Standing over Minyoung’s death, Yumin chose revenge. He sold his own identity into Seunggyu’s identity-laundering network and, before the laundering process was finished, faked his own death and disappeared — resurfacing only much later wearing Moonjae’s face and living his life. He funneled Minyoung’s identity into the same network, which is how a different, unidentified woman ended up in the Han River with a tattoo made to match hers.
Crew members pound urgently on the door mid-conversation. Yumin says only one of them is walking out of this. They fight. Moonjae stabs him and throws him through the broken window into the sea. Yumin’s last words as he goes under: “This isn’t over. You won’t be able to handle it.”
Who Is the Rat?
Sunyong is waiting at the cruise terminal. Moonjae walks off the ship wearing Yumin’s clothes, stepping into the fake’s identity even as he claims it’s gone for good — he tells Sunyong the impostor who tried to kill him fell into the sea during the struggle. Sunyong says he’s found Huisu. Moonjae, eager, tells him to run the DNA test right away. Sunyong asks him one question instead: “You killed the fake. You could have just gone back to being yourself. Why are you trying to become him instead?”
Moonjae goes to find Noja and offers to return the embezzled money. Before Noja can refuse, Moonjae tells him the truth he’s never told anyone — the fake identity, the false name, the fire he set at his old school — and admits all he ever wanted was to become someone better than that. Noja turns the money down anyway, warns him Seunggyu will still come looking for him, and heads back to his hometown to put his own past behind him.
Moonjae goes to his editor, Han Jihye. She hands him something Yumin left behind before he died: the unfinished manuscript for the second volume of The Rat. It contains everything that has happened so far. If the novel is completed, it will reveal that the real Rat was Moonjae all along. Moonjae tries to write his own version instead, but he can’t get a single line down. His fiction was never really his to begin with.
In his nightmares, a swarm of field mice eats away at his body. Outside the window, Yumin’s figure appears, watching him — whether he’s alive or a hallucination born of guilt is left deliberately unclear. As Moonjae starts to unravel, the doorbell rings sharply. A voice comes through the intercom: “Hey, rat. Open the door.” It’s Seunggyu. The series ends there.
Basic Info
Creators: Kim Hong-sun (director), Lee Jae-gon (writer)
Cast: Ryu Jun-yeol, Sul Kyung-gu, Lee Kyu-hyung
Based on: the webtoon Field Mouse by Ludovico
Inspired by: the Korean folktale “The Rat That Ate Fingernails”
Episodes: 10
Release: August 28, 2026, globally on Netflix
Everything you need to fully understand Netflix’s Mousetrap — what to know going in, the clues behind the ending, and how the original webtoon differs.
Mousetrap: What You Need to Know Before You Watch
Mousetrap Ending Explained — The Clues You Missed and Who the Real Rat Was
Mousetrap Original Webtoon Explained — The Full Story and How It Differs From Netflix
Looking for more Korean thrillers with this much twist density?
Should You Watch The East Palace? A Korean Viewer’s Take
Agent Kim Reactivated: What You Need to Know Before You Watch
A Shop for Killers Season 1: What You Need to Know Before Season 2
That final scene at the door — did it get you? I’m covering what it actually means, plus the clues planted early on, in the next post.
Some links in this post may be affiliate links. If you make a purchase through them, I may earn a small commission at no extra cost to you.